Drywell retrofit sump insert for storm water treatment

ABSTRACT

The present invention is a drywell retrofit sump insert designed to be placed inside an existing drywell or other similar Underground Injection Control (UIC). The drywell insert helps protect groundwater by removing pollutants from raw storm water runoff that enters the drywell before the drywell infiltrates the storm water into the ground. Once the raw storm water enters the drywell retrofit sump insert, sand, silt, grit, and other high-density pollutants settle out of the storm water stream and accumulate in the bottom of the drywell retrofit sump insert. Oil, grease, and other low-density pollutants float on top of the standing water inside the drywell retrofit sump insert and are kept inside by the oil baffles on the outlet holes. The drywell retrofit sump insert is comprised of a rigid cylinder with an integral flanged bottom attached by means of a watertight joint.

CROSS REFERENCE TO RELATED APPLICATIONS

61/462,245

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None

BACKGROUND OF THE INVENTION

In many parts of the country, there are no municipal storm sewer systems to convey storm water from a developed property to natural water bodies such as rivers and lakes. In these areas without storm sewers, drywells are sometimes used for the localized disposal of collected storm water from public roadways and private properties. Typically, these drywells are constructed of various products and materials, but almost all employ some kind of perforated tank or chamber which is installed underground in an excavated hole and backfilled with rocks or clean gravel.

The storm water from roof tops and paved areas is directed to the drywell through a grated opening at the pavement surface or via underground pipes. The storm water in the chamber then percolates through the perforations in the chamber, through the voids between the backfill rock, into the void spaces in the native soils, and eventually to the groundwater. This process is often times called “underground injection”.

Many such drywells receive polluted storm water that has not been treated before entering the drywell chamber. This polluted storm water is then infiltrated into the native soils and recharges the groundwater, potentially contaminating local drinking water supplies. Some of the common pollutants in roadway and parking lot runoff are oil, grease, and heavy metals. Fertilizers and pesticides from landscaped areas can also be washed into these drywell systems and eventually end up in the drinking water supply. In addition to the groundwater contamination issue, silt and sand washed into a drywell will settle out of the storm water and collect in the bottom of the chamber. Over time the function of the drywell can be impeded and eventually cease altogether if the pores in the adjacent native soils are plugged with silt. This failure of the drywell can result in localized flooding and property damage.

Governmental control and regulation of drywells has evolved with an increased understanding of groundwater contamination and potential environmental impact of pollutants injected into the ground through drywells. The U.S. Environmental Protection Agency (USEPA) regulates groundwater injection wells, which include drywells, under the auspices of the federal Safe Drinking Water Act in order to limit and control the injection of waste fluids in a manner that protects existing groundwater quality. In many states, the USEPA has delegated administration of an Underground Injection Control (UIC) program to state environmental agencies. Since 2000 the EPA has required that all new and existing groundwater injection wells be registered and authorized.

One of the primary requirements for authorization of storm water drywells and is that the drywell system must be designed and constructed to prevent pollutants such as petroleum products and heavy metals from contaminating groundwater supplies. The Federal Safe Water Drinking Act has no “grandfather” clause. Therefore, owners and operators of UIC systems must comply with current federal regulations regardless of when the UIC was constructed. The proposed invention facilitates the retrofit of certain drywell UIC's such that the flow of polluted storm water can be intercepted and some of the pollutants removed from the water prior to underground injection.

Drywells are often times constructed using a pair of perforated, precast concrete cylinders stacked vertically and topped with a precast concrete cone, and steel manhole lid. The perforated cylinders are typically four feet tall and four feet in diameter. The precast concrete cone is three feet tall and the diameter tapers from four feet at the bottom to two feet at the top. This form of drywell is created by first excavating a hole in the native soils to a depth such that the elevation of the steel manhole lid will match the finish pavement surface when site development work is completed. The two cylinder sections are stacked in the excavated hole on a concrete foundation, the cone with steel manhole lid is placed on top of the stacked cylinders, and the excavated hole is backfilled with clean rocks or gravel.

Typically the location of the drywell is such that the drywell will be suited for the collection of the storm water runoff. To assist this process, the finish pavement surface is usually graded such that the drywell is at the localized low point and a grated manhole lid is used to facilitate the collection of storm water into the drywell. The storm water then sheet flows across the pavement to the low point and enters the top of the drywell through the grated steel lid. This storm water flows directly into the drywell chamber, through the perforated holes of the concrete cylinders, through the drain rock and into the native soils with all the pollutants and contaminants that it may have accumulated as it flowed across the pavement surface.

The object of this invention is to create an efficient means of retrofitting existing drywells that do not have a system for removing sediment, oils and other pollutants from the storm water prior to infiltration with minimal disruption of the existing paved areas. The present invention is to be inserted inside the chamber of an existing drywell without the complete removal and replacement of the existing drywell and all of the cost associated with such a removal.

The objective of the present invention is to provide a simple, cost-effective means of retrofitting an existing drywell to add a water-quality treatment mechanism to remove pollutants from the storm water and facilitate the owner/operator's compliance with applicable water quality standards.

BRIEF SUMMARY OF THE INVENTION

The present invention is a drywell retrofit sump insert that is inserted inside the chamber of an existing drywell. The drywell retrofit insert simultaneously provides treatment of the entering storm water by two means. The first is that it creates a settling chamber for sand, grit, and other heavier-than-water particles to settle out and collect in the sump of the insert. The second treatment mechanism is the oil baffles on the outlet holes which trap most of the oil, grease, and other lighter-than-water hydrocarbons inside the retrofit insert. After this treatment by the retrofit insert, the water flows through the outlet holes to the annular space between the exterior face of the insert and the interior face of the drywell. The treated storm water is then able to pass through the perforations in the drywell chamber and percolate into the surrounding native soils. The drywell insert is comprised of a rigid cylinder whose diameter is roughly six inches less than the inside diameter of the existing drywell. This allows an annular space of roughly three inches around the circumference of the retrofit insert. The insert has a solid flange attached to its bottom by a watertight joint. The top of the insert cylinder has no lid or flange, in order that it may freely receive the storm water that falls into the drywell chamber through its grated lid. For those drywells that also receive storm water from an underground pipe, the sump insert can be fabricated with an optional pipe inlet stub. A pair of lifting lugs at the top of the insert cylinder facilitates its placement into the existing drywell chamber with a crane.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a cross sectional view of an existing drywell.

FIG. 2 is a section view of the drywell retrofit sump insert;

FIG. 3 is plan view of the present invention; and

FIG. 4 is a section view of a drywell after it has been retrofitted with the present invention (without optional pipe inlet stub or sampling port).

DETAILED DESCRIPTION OF THE INVENTION

Referring more particularly to the drawings, FIG. 1 shows a typical drywell 20 of the type to which this invention could be applied. The drywell chamber 21 is constructed of a pair of stacked perforated precast concrete cylinders 22 topped with a precast concrete cone 23 and a grated steel lid 24. The precast concrete components have been placed in a hole excavated into the native soils 25 and backfilled with drain rock 26. The adjacent paved surfaces 27 are typically constructed such that they are sloped causing storm water to run toward or drain directly into the drywell 20 through the grated steel lid 24. Once storm water is collected into the drywell chamber 21 the water is discharged through the perforations 22 a of the precast concrete cylinders 22. The storm water is then infiltrated through the drain rock 26 and into the native soils 25.

This particular invention is directed toward retrofitting previously constructed drywells of the type described above in order to provide a system for removing a portion of the pollutants from the storm water prior to percolating through the perforations 22 a of the precast concrete cylinders 22. The precast concrete cone 23 is replaced with a standard precast manhole barrel section 28 and a precast flat top 29 with centered hole to receive the steel frame and slotted lid 24.

The present invention, a drywell retrofit sump insert, is shown in FIGS. 2 and 3 with a section view of the device depicted in FIG. 2 and a top plan view of the device depicted in FIG. 3. The drywell retrofit sump insert is a rigid cylinder 30 with an integral flanged bottom 31 and two outlet holes 32 with oil baffles 33. Each of the oil baffles 33 is provided with a removable cleanout plug 34 and garbage screen 35.

Storm water or other run-off flows toward the drywell and enters the insert through a grated steel lid 24 that sits above the open top of the drywell insert cylinder 30. As water flows across the paved surface 27 toward the drywell, it is not uncommon for the water to collect or pick up sediment, oils and other pollutants prior to entering into the insert cylinder 30 through the grated steel lid 24 that sits above the insert cylinder 30.

When the storm water collects in the insert cylinder 30, the water level will remain fairly constant at the level of the outlet holes 32. The outlet holes are placed several feet above the top of the integral flanged bottom 31. By placing the outlet holes 32 several feet above the top of the integral flanged bottom 31, water falling into the insert cylinder 30 will collect allowing for sediment and pollutants that are heavier than water to settle out of the water prior to the water being discharged through the outlet holes 32 and into the annular space between the insert and the drywell chamber. The accumulated sediment 39 will collect at the bottom of the insert cylinder 30.

Several problems commonly associated with sediment-laden storm water are dealt with by removing the sediment from the storm water and trapping it in the insert chamber 30. Over time excess sediment can clog the perforations 22 a and reduce the efficiency of the infiltration system of the drywell 20. Trapping the sediment and heavier pollutants within the insert cylinder 30 can protect the continued function of the drywell, rather than allowing it to clog and fail. Also, contaminants such as heavy metals and phosphorous often attach to sediments in the storm water. By removing the sediment from the storm water, the attached pollutants are also removed from the storm water.

As the storm water collects in the insert cylinder 30, the lighter-than-water pollutants such as grease and oil naturally separate and rise to the surface of the collecting storm water. Aside from some seasonal fluctuation from evaporation, the water level 38 inside the sump insert remains fairly constant at the level of the outlet holes. The oil baffles 33 on the outlet holes 32 serve to trap most of these floating pollutants 40 within the insert cylinder 30.

The insert cylinder 30 is easily accessed for maintenance purposes by removing the grated steel lid 24 at the top of the manhole assembly. When required by the regulatory agency having jurisdiction, the sump insert can be fabricated with an optional sampling port 36 to allow for taking periodic grab samples of the treated storm water for laboratory testing. When the accumulated sediment 39 in the sump of the insert reach a level close to the garbage screens 35, the accumulated sediment 39 and accumulated floating pollutants 40 in the insert cylinder 30 can be pumped out and disposed of by a commercial septic service company.

In the infrequent event that a large rain storm should send storm water flowing into the sump insert cylinder 30 faster than it could flow out of the outlet holes 32, the storm water would bypass the outlet holes 32 by filling to the top and spilling over the top of the insert cylinder 30 and spilling into the annular space between insert and drywell.

FIG. 4 shows a completed installation of the drywell insert placed within the two perforated precast concrete cylinders 22.

A retrofit installation of the drywell retrofit sump insert would require: excavation of the pavement around the drywell grated steel lid 24; removing the grated steel lid 24 and steel frame 24; accessing and removing the precast concrete cone 23; placing the drywell insert inside the perforated precast concrete cylinders 22; installing a non-perforated precast manhole section 28 on top of the upper perforated precast cylinder, and installing a precast flat manhole top 29 with a centered hole for the steel frame and lid. The precast manhole section 28 and the precast flat manhole top 29 are both readily available commercial items. The steel frame and grated steel lid 24 could be salvaged from the precast concrete cone 23 for reinstallation on the drywell flat top 29. The backfill material removed around the precast concrete cone 23 is returned and the sloped pavement 27 is rebuilt.

Parts List for Drywell Retrofit Sump Insert

-   20. Typical existing drywell -   21. Drywell chamber -   22. Perforated precast concrete cylinders -   22 a. Perforations -   23. Precast concrete cone -   24. Grated steel lid and frame -   25. Native soils -   26. Drain rock -   27. Paved surface -   28. Precast concrete manhole section -   29. Precast flat manhole top -   30. Rigid cylinder -   31. Integral flanged bottom -   32. Outlet holes -   33. Oil baffles -   34. Cleanout plugs -   35. Garbage screens -   36. Optional sampling port -   37. Optional pipe inlet stub -   38. Typical water level -   39. Accumulated sediment -   40. Accumulated floating pollutants 

1. A drywell retrofit sump insert for treating storm water runoff comprising; a hollow, rigid cylinder, the top end of which is open without lid or cover and the bottom end of which has an integral water-tight cover with flange extending beyond the diameter of the cylinder wall, one or more outlet holes in the wall of the cylinder each with a screened baffle; a portion of the cylinder's interior space is dedicated to the collection of lighter-than-water pollutants and particles; a portion of the cylinder's interior space dedicated to the collection of heavier-than-water pollutants and particles; a portion of the cylinder's interior space contains standing water; a means for attaching lifting devices to the cylinder wall for moving the drywell retrofit sump insert
 2. The drywell retrofit sump insert of claim 1 wherein the diameter of the cylinder and the diameter of the integral flanged bottom are sized to readily fit inside the perforated chamber of an already existing drywell and to maintain an annular void space between the exterior face of the drywell retrofit sump insert and the interior face of the existing drywell.
 3. The drywell retrofit sump insert of claim 1 wherein the height of the rigid cylinder is sized to maximize the volume within the cylinder and yet allow an appropriate air gap between the open top end of the cylinder and the underside of the drywell lid structure for the overflow of storm water should the outlet holes become plugged.
 4. The drywell retrofit sump insert of claim 1 wherein the top end of the cylinder is open to facilitate access for maintenance purposes and to receive raw storm water falling vertically through a grated opening at the top of the drywell structure, typically installed flush with the ground surface.
 5. The drywell retrofit sump insert of claim 1 wherein the cylinder wall may be fitted with one or more circular openings to receive raw storm water conveyed to the pre-existing drywell by underground pipes.
 6. The drywell retrofit sump insert of claim 1 wherein each of the outlet holes in the cylinder wall shall have an outlet baffle to trap the floating lighter-than-water pollutants inside the cylinder.
 7. The drywell retrofit sump insert of claim 1 wherein the top surface of each outlet baffle shall have a removable plug for maintenance purposes.
 8. The drywell retrofit sump insert of claim 1 wherein the top surface of an outlet baffle may be fitted with an access port through which samples of the treated storm water can be drawn and tested.
 9. The drywell retrofit sump insert of claim 1 wherein the opening at the bottom of each outlet baffle shall be screened to trap garbage and other large particles inside the cylinder.
 10. The drywell retrofit sump insert of claim 1 wherein the materials used to make the various components of the drywell retrofit sump insert include; metal, plastic, fiberglass, concrete, or a combination of these materials.
 11. The method of retrofitting a drywell with a drywell retrofit sump insert, comprising; excavating around a pre-existing drywell; exposing the precast concrete cone of an already existing drywell; removing the precast cone; cleaning and leveling the floor of the pre-existing drywell; fabricating a steel vessel comprised of a hollow, rigid cylinder, the top end of which is open without lid or cover and is readily able to accept raw storm water, the bottom end of which has an integral water-tight cover which creates a dedicated space within the cylinder for heavier-than-water pollutant particles to settle out and accumulate, this same bottom cover having a flange extending beyond the diameter of the cylinder wall to maintain an annular void space between the exterior face of the fabricated cylinder and the interior face of the pre-existing drywell, one or more outlet holes in the wall of the cylinder for the outflow of the treated storm water, each of the outlet holes fitted with a screened baffle to keep the floating lighter-than-water pollutants inside the cylinder; placing the fabricated steel vessel inside perforated precast concrete cylinder of the pre-existing drywell; replacing the removed precast concrete cone with a non-perforated precast concrete cylinder section and a flat precast concrete manhole top; installing a grated lid on top of the flat precast concrete manhole top; backfilling the excavated soil materials and replacing any removed pavement. 